Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material may be ionized in an ion source, the ions may be accelerated to form an ion beam of prescribed energy, and the ion beam may be directed at a surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity. The ion beam may be distributed over the wafer area by beam scanning, by wafer movement or by a combination of beam scanning and wafer movement. The energetic ions in the ion beam are charged particles. Hence, in some instances the front surface of the wafer may accumulate an excessive charge that may lead to damage of the devices on the wafer.
One cause of excessive charge accumulation may be insufficient electrical contact of platen grounding pins due to improper installation, damage, wear, contamination, or an oxide or other insulting layer on the rear surface of the wafer. The platen grounding pins should normally contact the rear surface of the wafer when the wafer is properly supported by the platen in order to provide a path to ground for charge neutralization. Another cause of excessive charge accumulation may be the non-use or insufficient use of an electron supply source when a positive charge accumulates on the wafer. The electron supply source, e.g., a plasma flood gun (PFG), may supply low energy electrons to neutralize an accumulated positive charge on the wafer. In addition, isolated areas on the front surface of the wafer, e.g., an oxide layer, that do not allow charge to dissipate through the wafer substrate may cause charge accumulation.
Hence, wafer charge monitoring techniques for monitoring charge accumulation have been developed. One conventional approach utilizes a measuring electrode disposed in a substrate-holding unit that is not directly exposed to the ion beam. Yet another conventional approach utilizes a charge-receiving conductor disposed on a front surface of a rotating wafer disk that supports a plurality of wafers. The charge-receiving conductor is coupled to another conductor disposed on the rear surface of the rotating wafer disk. The conductor on the rear surface of the rotating wafer disk is then capacitively coupled to yet another conductor fixed to a disk chamber. A drawback of this approach is that it requires a plurality of conductors and capacitive coupling of at least two of the conductors. In addition, this technique requires a spinning wafer disk for batch implantation, which is not suitable for single wafer ion implantation.
Accordingly, there is a need for new and improved methods and apparatus for monitoring charge accumulation on a wafer during ion implantation.